To operate a heat source machine, for example, a chiller on the design value, it is necessary to manage a flow rate of a heat transfer medium (chilled water) flowing into an evaporator, but a flow meter for measuring the flow rate of the heat transfer medium may not be provided in the chiller because a flow meter for measuring a flow rate is expensive, and it is required to reduce the number of components and so on.
In a chiller that is not provided with a flow meter for measuring the flow rate of a heat transfer medium, for example, the flow rate of the heat transfer medium to be used in computation for controlling the degree-of-opening of an expansion valve diverges from the actual flow rate of the heat transfer medium, and problems such as those described below may occur due to the deviation between the degree-of-opening of the expansion valve calculated by the computation and the actually required degree-of-opening of the expansion valve.
If the degree-of-opening of the expansion valve is excessive, the flow rate of the refrigerant becomes excessive, and thus, it is possible that the coefficient of performance (COP (coefficient of performance)) of a chiller deteriorates as a result of the motor power input of the chiller becoming excessive, that so-called refrigerant flood back occurs, where the compressor draws in liquid-phase refrigerant, and that gas bypass occurs, where a portion of the refrigerant flows into an evaporator in the gas phase without being turned into the liquid phase in a condenser due to insufficient subcooling at the condenser.
On the other hand, if the degree-of-opening of the expansion valve is too small, the pressure difference between the condenser and the evaporator becomes excessive, and thus, it is possible that, the COP deteriorates as a result of the motor power input of the chiller becoming excessive.
Therefore, as the technologies for measuring a flow rate, PTL 1 discloses the estimation system of cooling water flow rate in that a chilling load is computed based on measurement values of an outlet, temperature of chilled water, an inlet temperature of the chilled water and a flow rate of the chilled water, a heat exchange coefficient is computed based on the inlet temper at lire of the chilled water and the chilling load, and a flow rate of a cooling water is derived from measurement values sent front, a group of sensors and the heat exchange coefficient, and then output it.
PTL 2 describes the technology in that, for a plurality of air conditioning machines, a plurality of differential pressure sensors are provided to measure a differential pressure between an inlet and an outlet of chilled and heated water in each of the plurality of air conditioning machines and a flow sensor is provided to measure the entire flow rate of the chilled and heated water, and by providing a flow path allowing only one differential pressure sensor to operate through valve switching and the like, the relation between the flow rate and the differential pressure is obtained before operation of cooling, and on the operation of cooling, a flow rate of the chilled and heated water is obtained using the differential pressure sensors.